In MEMS capacitive acceleration sensors, angular velocity sensors, angle sensors, and other sensors, a capacitance-to-voltage (CV) conversion amplifier is used. The CV conversion amplifier converts a change ΔC of a capacitance value generated in an MEMS capacitive element into a voltage signal ΔV. Since the CV conversion amplifier is located in the first stage of a signal detection circuit, the CV conversion amplifier is required to have a sufficiently low noise. In order to achieve this, a large consumption current is necessary.
In order to relax the noise specifications of the subsequent circuit block and later ones, it is necessary to increase a capacitance-to-voltage conversion gain ΔV/ΔC as large as possible. However, conventionally, a problem arises in that an increase in the capacitance-to-voltage conversion gain causes a considerable decrease in the amplitude range of the output voltage of the CV conversion amplifier, or causes the faulty operation of the CV conversion amplifier.
In other words, in the case of a pseudo-differential CV conversion amplifier using two single-ended output operational amplifiers in parallel, an increase in the capacitance-to-voltage conversion gain causes the center voltage level of the output of each of the single-ended output operational amplifiers to be considerably shifted from a desired value, which is typically about a half of a power supply voltage. As a result, the amplitude range of the output voltage of the CV conversion amplifier is considerably decreased.
In the case of a fully-differential CV conversion amplifier using one fully-differential operational amplifier, an increase in the capacitance-to-voltage conversion gain causes the in-phase potential (the average potential) of the differential input of the fully-differential operational amplifier to be considerably shifted from a desired value. As a result, this causes the faulty operation of the CV conversion amplifier. When the amplitude range of the output voltage of the CV conversion amplifier is deceased, the tolerance of the input signal of the sensor is narrowed. For example, in the acceleration sensor, the range of input acceleration signals, which are normally detectable, is narrowed.
Therefore, conventionally, a capacitance for adjusting the in-phase potential is added to the input node of the operational amplifier, and hence the amplitude range of the output of the CV conversion amplifier is secured, or normal operating states are secured. Such configurations added with a capacitance for adjusting the in-phase potential are described in Patent Literature 1, Nonpatent Literature 1, or Nonpatent Literature 2, for example.